The present invention applies in particular, but not exclusively, to the cellular mobile telephone domain, such as that provided for in the Global System for Mobile Communications (GSM) standard. The means of propagation of the digital signal can be air in the case of cellular mobile telephones, or else any other means of propagation such as a cable, for example, in other applications.
A fundamental factor limiting the performance of a digital communication system is the phenomenon known as “inter-symbol interference.” Such inter-symbol interference causes, at the receiver level, temporal occupation of each symbol transmitted (e.g., a bit) that is longer than the initial duration of the symbol (also referred to as the “bit time,” for example). Stated otherwise, the signal received at a given instant does not depend on one symbol alone (e.g., a bit) but also on the other bits or symbols sent which extend over durations greater than those of a bit time.
In practice, the signal received at a given instant depends on the symbol concerned, and also on the adjacent symbols. There are several causes of inter-symbol interference. One of them is due in particular to the multiple propagations of the signal between the sender and the receiver when the signal is reflected or diffracted by various obstacles which, upon reception, leads to several signal copies mutually shifted in time.
Other causes disturb the analysis of the symbols received. These may include additions of miscellaneous signals and noise, such as thermal noise, miscellaneous transmissions from other senders, and most of the other interference caused by the other senders (e.g., GSM) using the same frequency band at the same moment or else an adjacent band. Moreover this interference between symbols is produced not only by the means of propagation between the sender and the receiver but also by the sending/receiving devices themselves (modulator, filter, etc).
During communications with interference between symbols, the problem of estimating the impulse response of the transmission channel may arise. The quality of this estimate depends on one's capacity to eliminate the interference between symbols, and hence to make correct decisions regarding symbols sent. Generally, the estimate of the impulse response of the channel, or more simply the “channel estimate,” is effected within the GSM telephone domain. In particular, this may be done by using least squares techniques and by using a predetermined sequence of symbols known to the sender and to the receiver. This is commonly referred to as a “training sequence.” This training sequence is present within each symbol train (or “burst”) sent.
When the characteristics of the channel are sufficiently well estimated, the estimated coefficients of the impulse response of the channel are used in a so-called “equalization” processing operation, as will be appreciated by those of skill in the art. This is done to decode the signal received, i.e., to retrieve the logic values of the bits (data) sent in the train. The equalization processing operation is conventionally followed by the so-called “channel decoding” processing operations for reconstructing the information (e.g.; speech) initially coded at the sender.
There are numerous equalization algorithms which are well known to the person skilled in the art. Among these equalization processing operations, two major classes are considered herein. This first class is those operations which carry out detection symbol by symbol, such as the algorithm known as decision feedback equalization (DFE), for example, the essential aspects of which are described in “Digital Communications” by John G. Proakis, third edition, McGraw-Hill, Inc. The second class is those operations which carry out the detection of a sequence of symbols, such as the algorithms known as maximum likelihood sequence estimation (MLSE) or decision feedback sequence estimation (DFSE), for example. These two algorithms are the subject of numerous publications. For example, additional reference regarding the MLSE algorithm is provided in the work by John G. Proakis cited above, and for the DFSE algorithm additional reference is provided in an article by Hans C. Guren and Nils Holte entitled “Decision Feedback Sequence Estimation for Continuous Phase Modulation on a Linear Multipath Channel,” IEEE Transactions on Communications, Vol. 41, No. 2, February 1993.
The symbol by symbol detection algorithms have low complexity relative to the sequence-based detection algorithms, but they give inferior performance. This is why equalization algorithms using sequence-based estimation are generally preferred. However, the MLSE algorithm, which is an optimal algorithm, may be difficult to implement because of its complexity in certain applications, such as quadrature modulations (“M-ary” modulations). In particular, these may include a QPSK modulation (2 bits per symbol) or 8PSK modulation (3 bits per symbol), as will be understood by those of skill in the art.
In such applications, the DFSE algorithm is then preferably used. This also makes it possible to carry out the sequence detection while reducing, for example, the number of states of the trellis used in this algorithm by a decision feedback mechanism. Lower complexity is then obtained, and hence easier implementation, but lower performance.